Potato psyllid

Potato psyllids can spread the Liberibacter bacterium that causes zebra chip disease in potatoes. Psyllids have been found in a few Washington state fields.

MOSES LAKE, Wash. — More potato psyllids have been spotted earlier than usual, and Carrie Wohleb is warning farmers to be on the lookout.

“The first potato psyllid we find in potato fields makes a big difference,” said Wohleb, a Washington State University Extension vegetable crop specialist. “The earlier we see them, usually the larger population we end up with at the end of the season.”

The insects are the only vector for the zebra chip disease, which causes dark stripes in potatoes and destroys their value. So far, none of the psyllids Wohleb has collected tested positive for the bacteria that causes the disease.

Potato psyllids are easy to miss when scouting and, because the symptoms of zebra chip take time to develop, the damage may be done before growers are aware that their crop is at risk, according to WSU.

“We really can’t tell if an infected psyllid is going to enter your field or if it’s a harmless psyllid, so we consider them all harmful,” she said.

Each week Wohleb monitors 45 fields from Quincy to the Oregon border as part of an insect monitoring network.

Psyllids were found in 40-50% of the fields, she said.

Historically, the region has a low infection rate for psyllids with the bacterium Candidatus liberibacter solanacearum, which causes zebra chip. The disease was first spotted in the U.S. in 2004.

In 2016, the monitoring network found and tested 21,000 psyllids and farmers sent in an additional 10,000. Of those, two were infected with zebra chip, Wohleb said.

Potato processors reported scattered instances of the disease that year.

The next year, Wohleb collected 152 psyllids.

“So the population jumps are huge,” she said. “We’re dealing with crazy sample sizes when a fraction of the population is impacted, but you don’t want to be the one unlucky grower that has the infected psyllid.”

Zebra chip wiped out several fields in the region in 2011. Those “huge, devastating losses” haven’t recurred, Wohleb said, indicating farmers are doing a good job keeping the pest and disease on their radar.

USDA researchers believe monitoring the perennial matrimony vine, a relative to the potato, can indicate how well the psyllid population survived the winter.

They found psyllids back in March, which is earlier than usual and also could indicate a larger population, Wohleb said.

“I think we still don’t understand everything there is to know about how a potato psyllid overwinters and how successful it’s going to be and what it is about this season that makes psyllids breed more and spread more,” she said.

Wohleb and other researchers will continue monitoring for psyllids and other insects and putting out weekly pest alerts.

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